Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide systems and methods for regulating output currents. Merely by way of example, some embodiments of the invention have been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) are widely used for lighting applications. Oftentimes, approximately constant currents are used to control working currents of LEDs to achieve constant brightness. FIG. 1 is a simplified diagram showing a conventional LED lighting system. The LED lighting system 100 includes a controller 102, resistors 108, 116, 122, 124 and 128, capacitors 106, 110, 112 and 130, a full-wave rectifying component 104, diodes 114 and 118, an inductive component 126 (e.g., an inductor), and a Zener diode 120. The controller 102 includes terminals (e.g., pins) 138, 140, 142, 144, 146 and 148.
An alternate-current (AC) voltage 150 is applied to the system 100. The rectifying component 104 provides an input voltage 152 (e.g., a rectified voltage no smaller than 0 V) associated with the AC voltage 150. The capacitor 112 (e.g., C3) is charged in response to the input voltage 152 through the resistor 108 (e.g., R1), and a voltage 154 is provided to the controller 102 at the terminal 148 (e.g., terminal VDD). If the voltage 154 is larger than a threshold voltage (e.g., an under-voltage lock-out threshold) in magnitude, the controller 102 begins to operate, and a voltage associated with the terminal 148 (e.g., terminal VDD) is clamped to a predetermined voltage. The terminal 138 (e.g., terminal DRAIN) is connected to a drain terminal of an internal power switch. The controller 102 outputs a drive signal (e.g., a pulse-width-modulation signal) with a certain frequency and a certain duty cycle to close (e.g., turn on) or open (e.g., turn off) the internal power switch so that the system 100 operates normally.
If the internal power switch is closed (e.g., being turned on), the controller 102 detects the current flowing through one or more LEDs 132 through the resistor 122 (e.g., R2). Specifically, a voltage 156 on the resistor 122 (e.g., R2) is passed through the terminal 144 (e.g., terminal CS) to the controller 102 for signal processing during different switching periods associated with the internal power switch. When the internal power switch is opened (e.g., being turned off) during each switching period is affected by peak magnitudes of the voltage 156 on the resistor 122 (e.g., R2).
The inductive component 126 is connected with the resistors 124 and 128 which generate a feedback signal 158. The controller 102 receives the feedback signal 158 through the terminal 142 (e.g., terminal FB) for detection of a demagnetization process of the inductive component 126 to determine when the internal power switch is closed (e.g., being turned on). The capacitor 110 (e.g., C2) is connected to the terminal 140 (e.g., terminal COMP) which is associated with an internal error amplifier. The capacitor 130 (e.g., C4) is configured to maintain an output voltage 196 to keep stable current output for the one or more LEDs 132. A power supply network including the resistor 116 (e.g., R5), the diode 118 (e.g., D2) and the Zener diode 120 (e.g., ZD1) provides power supply to the controller 102.
FIG. 2 is a simplified conventional diagram showing the system controller 102 as part of the system 100. The system controller 102 includes a ramp-signal generator 202, an under-voltage lock-out (UVLO) component 204, a comparator 206, a logic controller 208, a driving component 210 (e.g., a gate driver), a power switch 282, a demagnetization detector 212, an error amplifier 216, and a current-sensing component 214. For example, the power switch 282 includes a bipolar junction transistor. In another example, the power switch 282 includes a MOS transistor. In yet another example, the power switch 282 includes an insulated-gate bipolar transistor.
As shown in FIG. 2, the UVLO component 204 detects the signal 154 and outputs a signal 218. If the signal 154 is larger than a first predetermined threshold in magnitude, the system controller 102 begins to operate normally. If the signal 154 is smaller than a second predetermined threshold in magnitude, the system controller 102 is turned off. The second predetermined threshold is smaller than or equal to the first predetermined threshold in magnitude. The error amplifier 216 receives a signal 220 from the current-sensing component 214 and a reference signal 222 and outputs an amplified signal 224 to the comparator 206. The comparator 206 also receives a signal 228 from the ramp-signal generator 202 and outputs a comparison signal 226. For example, the signal 228 is a ramping signal and increases, linearly or non-linearly, to a peak magnitude during each switching period. The logic controller 208 processes the comparison signal 226 and outputs a modulation signal 230 to the driving component 210 which generates a drive signal 280 to open or close the switch 282 (e.g., at the gate terminal). The switch 282 is coupled between the terminal 138 (e.g., terminal DRAIN) and the terminal 144 (e.g., terminal CS). In addition, the logic controller 208 outputs the modulation signal 230 to the current-sensing component 214. For example, the demagnetization detector 212 detects the feedback signal 158 for determining the beginning and/or the end of a demagnetization process of the inductive component 126 and outputs a trigger signal 298 to the logic controller 208 to start a next cycle. The system controller 102 is configured to keep an on-time period associated with the comparison signal 226 approximately constant for a given output load so as to achieve high power factor and low total harmonic distortion.
The system controller 102 is operated in a voltage-mode where, for example, the signal 224 from the error amplifier 216 and the signal 228 from the oscillator 202 are both voltage signals and are compared by the comparator 206 to generate the comparison signal 226 to drive the power switch 282. Therefore, an on-time period associated with the power switch 282 is affected by the signal 224 and the signal 228.
Under stable normal operations, an average output current is determined, according to the following equation (e.g., without taking into account any error current):
                                          I            o                    _                =                              V            ref_ea                                R            cs                                              (                  Equation          ⁢                                          ⁢          1                )            where Vref_ea represents the reference signal 222 and Rcs represents the resistance of the resistor 122. As shown in Equation 1, the parameters associated with peripheral components, such as Rcs, can be properly selected through system design to achieve output current regulation.
For LED lighting, efficiency, power factor and total harmonic are also important. For example, efficiency is often needed to be as high as possible (e.g., >90%), and a power factor is often needed to be greater than 0.9. Moreover, total harmonic distortion is often needed to be as low as possible (e.g., <20%) for some applications. But the system 100 often cannot satisfy all these needs.
Hence it is highly desirable to improve the techniques of regulating output currents of power conversion systems.